I don't even consider Zevran, and I do think Alistair is cool, I find him to be a funny companion. However, I'm playing a male character, so he isn't even an option. Besides, I already chose Leliana, and I have to say I think she probably is the best romance option in the game. Although I do wonder a few things about Leliana's past...
Interesting discussion on Leliana v Morrigan
I've romanced both. Morrigan comes across initially as someone who does not want anything but a physical relationship but ultimately confesses to an emotional attachment. However her ruthlessness is such that she does not hesitate to suppress it in favour of her agenda whatever that maybe. Even in the Witchunt DLC Morrigan is deliberately evasive about that but does welcome your company and commitment then reaffirming her own feelings about you - well that's what she says anyway
Leliana? Warm trustworthy and comforting - but she also felt that way about Marjolaine - once! Leliana also has her own personal agenda in my view which has more to do with the Chantry's interests rather than your own objectives. Personally I have no doubt that if your character chose a path that went contrary to the Chantry's she would become at least your political opponent if not ultimately your antagonist
Certain aspects of Morrigan's character are reminiscent of the Viconia Devir character and romance from BG - another epilogian tragedy. Viconia is described as Neutral Evil but she is more amoral than anything - her actions like Morrigans are shaped by her background, upbringing and the society she was raised in